1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same and is suitable for, for example, a television camera for broadcasting, a movie camera, a video camera, a digital still camera, a surveillance camera and a camera for silver-halide photographs.
2. Description of the Related Art
In recent years, for image pickup apparatuses such as a television camera, a movie camera, a camera for silver-halide films, a digital camera and a video camera, there is a need for a zoom lens having a wide angle of field, a high zoom ratio and high optical performance in a wide zoom range. Various zoom lenses having wide angles of view and high zoom ratios have been known. For example, a so-called four-unit zoom lens has been known, which includes, in order from an object side to an image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power for magnification-varying, a third lens unit having positive refractive power for correcting image plane variation caused by magnification-varying, and a fourth lens unit having positive refractive power for imaging (Japanese Patent Application Laid-Open No. 2001-356381).
Japanese Patent Application Laid-Open No. 2001-356381 discloses a four-unit zoom lens having a zoom ratio of about 2.5 and an angle of shooting view at a wide-angle end of about 30°.
It is relatively easy to provide a wider angle of field of the four-unit zoom lens having the lens configuration described above. To realize a further wide angle of field in the four-unit zoom lens, it is necessary to intensify the refractive power of the first lens unit. However, if the refractive power of the first lens unit is intensified, variation in aberrations in zooming increases. This makes it difficult to obtain high optical performance in the entire zoom range.
On the other hand, if a plane subject to a high-order aberration is intentionally provided in the fourth lens unit having a relatively high degree of freedom of a lens configuration, it is easy to suppress aberration variation in zooming and keep optical performance over the entire zoom range in a well-balanced state. In general, a lens surface having high refractive power is necessary to generate a high-order aberration. Consequently, providing the plane subject to a high-order aberration in an optical system results in an increase in sensitivity, which is a change in optical performance with respect to a change in the position of the provided lens surface. This makes it difficult to manufacture and assemble the zoom lens.
To make it easy to manufacture the four-unit zoom lens and obtain high optical performance over the entire zoom range while realizing a wider angle of field, it is important to appropriately design, in particular, the lens configuration of the fourth lens unit that does not move for zooming and the shapes and the like of the respective lenses included in the fourth lens unit. If these configurations are inappropriately specified, it is difficult to manufacture the zoom lens and obtain high optical performance over the entire zoom range while realizing a wider angle of field.